1. Field of the Invention
This invention relates generally to the field of digital circuit design and, more particularly, to the design of a driver circuit to power a fan.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs) with relatively high operating temperatures.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's rotational speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so, or in certain systems, such as personal computers (PCs) for example, lowering the rotational speed of the fan and allowing the fan to continue running at a minimum rotational speed.
Some fan drive systems use a signal generator that provides a Pulse Width Modulated (PWM) signal to drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the rotational speed of the fan. Signal generators that provide PWM signals are useful because they provide a digital control for the pulse width of a signal. The fan is typically powered only for the duration of the pulse. Between pulses power to the fan is turned off, although the fan is typically still spinning during this time. The duty cycle of the PWM pulse train presently being provided to the fan determines the fan's rotational speed. Another typical way to control three-wire fans is to drive the fan by utilizing a high side Field Effect Transistor (FET), thereby controlling the DC voltage supplied to the fan. Generally, this provides an effective dynamic control range of 3V, which typically ranges from 5V down to around 2V. The lower limit voltage (2V) is still sufficient to power the fan circuitry, and valid tachometer signals may still be obtained from the fan.
In some instances, alternatives to a PWM generator may be preferred for driving the fan, while retaining digital control of the fan. For example, certain systems may best be configured using fans that are powered by a linear control voltage, while other systems may be configured with fans that are pulse powered, but an alternative to a PWM generator is still preferred. In such cases, it may be desirable to provide a driver circuit that is capable of providing either pulses or a linear voltage to power the fan, while also generating the pulses or the linear voltage based on digital control signal or signals. In addition, with ever increasing requirements for smaller die size and accuracy, it may be desirable to design a fan driver circuit that can provide a highly power signal to the fan, and can be implemented with standard digital design techniques, which would allow for both compactness due to smaller size, and higher accuracy due to better testability.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.